In recent years, in a long distance optical communication in particular, a digital coherent-type optical transmission system has been developed that can dramatically increase the communication capacity per one channel and has been increasingly put into commercial use. The digital coherent-type communication generally uses a polarization multiplexing method to give different signals to two polarizations orthogonal to each other to double the transmission capacity.
FIG. 1 illustrates the configuration of an optical reception circuit (optical polarization separation/demodulation circuit) 9100 of a typical digital coherent polarization multiplexing method according to the related art. A coherent-type optical transmission circuit and an optical reception circuit are characterized in having a light source also at the reception side so that the standard light (reference light) inputted from this light source is allowed to interfere with a signal light inputted from a transmission path to thereby detect the signal with a higher sensitivity.
In FIG. 1, the arrow shown by the solid line at the left end shows the standard light inputted from the light source while the arrow shown by the dotted line shows the signal light inputted from the transmission path. FIG. 1 illustrates a polarization beam splitter 9101, a polarization rotator 9102, the second optical power splitter 9103, a coherent optical mixer 9104 functioning as the first light demodulation circuit, a coherent optical mixer 9105 functioning as the second light demodulation circuit, and photo detectors 9106 and 9107.
The polarization multiplexed signal light (dotted line) inputted from the transmission path is received by light receptor 9100 and is separated by the polarization beam splitter 9101 to TE polarized light and TM polarized light components. The continuous light (solid line) of the TE polarized light functioning as the standard light is inputted from a not-shown light source and is branched by the second optical power splitter 9103 to two components. The TE polarized light component of the separated signal light and the standard light of one branched TE polarized light are demodulated by the coherent optical mixer 9104. The TM polarized light component of the separated signal light is converted by the polarization rotator 9102 to TE polarized light. The resultant TE polarized light and the standard light of the other branched TE polarized light are inputted to the coherent optical mixer 9105 and are demodulated. The demodulated light signal is converted by the photo detectors 9106 and 9107 to a reception electric signal and the resultant signal is outputted.
The polarization beam splitter 9101, the polarization rotator 9102, the second optical power splitter 9103, and the coherent optical mixers 9104 and 9105 are generally realized on one chip by a waveguide-type light integrated circuit. The waveguide is composed of material such as semiconductor such as quartz or indium phosphide or silicon. A semiconductor or silicon light integrated circuit is also realized by being integrated to include the photo detectors 9106 and 9107.
Each of the coherent optical mixers 9104 and 9105 is a light circuit having 2 inputs and 4 outputs in which the maximum interference is obtained when two inputted light waves have a phase angle difference of 0, 90, 180, and 270 degrees. Interference light waves are outputted depending on the phase angle difference from the respective 4 outputs. The coherent optical mixers 9104 and 9105 have a light demodulation characteristic that is determined based on a level (or a phase error) at which the phase angle difference actually causing the maximum interference is displaced from phase angle differences of 0, 90, 180, and 270 degrees causing the maximum interference from the design viewpoint. A coherent optical mixer actually applied to an optical reception circuit must have a phase error lower than a specified standard value. Currently, a general phase error is specified to be ±5 degrees or less.
This phase error is a characteristic that is easily influenced by a manufacture variation. Thus, in the manufacture of a coherent optical mixer, the phase error must be evaluated and a circuit satisfying a set standard must be selected. The phase error is evaluated generally using a delay circuit.
FIG. 2 is a plan view illustrating the configuration of a chip 9210 of a coherent optical mixer circuit in which a delay circuit is added for the evaluation of the phase error. FIG. 2 illustrates the coherent optical mixer circuit 9104 of FIG. 1, an input waveguide 9200, a light splitter 9201, the first and second arm waveguides 9202 and 9203 constituting the delay circuit, two input waveguides 9204 and 9205 to the coherent optical mixer circuit 9104, and four interference light outputs 9206, 9207, 9208, and 9209. When the delay circuit portion of the chip is cut away after the evaluation of the phase error, the chip is cut apart along a cutaway standard line 9211 (shown as a dashed dotted line).
FIG. 3 is a plan view illustrating one example of the waveguide configuration of the coherent optical mixer circuit 9104 of FIG. 2. In FIG. 3, the two input waveguides and the four output waveguides are designated by the same reference numerals as those of FIG. 2.
In FIG. 3, the interference portion of the coherent optical mixer circuit 9104 is composed of a 1×2 optical coupler 9301, three 2×2 optical couplers 9302, 9304, and 9305, and four waveguides 9303 having an equal length for connecting them.
FIG. 4 is a plan view illustrating another example of the waveguide configuration of the coherent optical mixer circuit 9104 of FIG. 2. In FIG. 4, the two input waveguides and the four output waveguides are designated by the same reference numerals as those of FIG. 2.
In FIG. 4, the interference portion of the coherent optical mixer circuit 9104 is composed of a single multimode interference circuit (Multi-Mode Interferometer: MMI) 9401. An interference light output is obtained based on predetermined phase difference conditions determined by the differences of the light paths from two input ports to the four outputs ports, respectively, of the MMI chip 9401 to which the signal light and the reference light are inputted. A coherent optical mixer configured by a single MMI has been conventionally reported, an example of which is described in detail in the following Non Patent Literature 1.
FIGS. 5A and 5B illustrate the transmission spectra of monitor lights outputted from the respective four output waveguides 9206, 9207, 9208, and 9209 when monitor light is inputted from the input waveguide 9200 for the evaluation of the phase error in the coherent optical mixer circuit in the chip 9210 added with the delay circuit of FIG. 2. FIGS. 5A and 5B illustrate that the spectra shown by OUT #1, OUT #2, OUT #3, OUT #4 illustrate the transmission spectra of the monitor lights to the output waveguides 9206, 9207, 9208, and 9209, respectively.
The inputted monitor light for the phase error evaluation is branched to two lights. The resultant two lights are inputted to the MMI with a delay time difference given in the delay circuit and are interfered and outputted. Thus, the configuration of FIG. 2 forms a Mach-Zehnder interference circuit and shows a frequency characteristic (light transmission spectrum) having a cyclic drop determined by the delay time difference.
As shown in FIGS. 5A and 5B, the monitor lights outputted from the four respective output ports of the MMI basically have the same shape of light transmission spectrum. However, the light transmission spectra are displaced by about ¼ cycle in the frequency axis direction depending on the interference phase difference conditions of the four output ports.
FIG. 5A shows the monitor light transmission spectra when the coherent optical mixer circuit shown in FIG. 3 is used. FIG. 5B shows the monitor light transmission spectra when the coherent optical mixer circuit shown in FIG. 4 is used.
The phase error is evaluated by analyzing these transmission spectra. Specifically, when the interference conditions at the four outputs completely match the phase angle differences of 0, 90, 180, and 270 degrees, the maximum transmission frequency (or the maximum quenching frequency) of the transmission spectra of the respective outputs are arranged to have an equal interval in the light frequency axis direction. Thus, the phase error can be obtained by confirming the frequency array having an equal interval and the actual displacement of the transmission (or quenching) frequency.
In FIG. 5A, showing the spectra of the coherent optical mixer circuit of FIG. 3, OUT #1 and OUT #2 adjacent to each other and OUT #3 and OUT #4 adjacent to each other have the phase angle differences of the interference conditions relatively different from each other by 180 degrees, respectively. On the other hand, in FIG. 5B, showing the spectra of the coherent optical mixer circuit of FIG. 4, OUT #1 and OUT #4, OUT #2 and OUT #3 have the phase angle differences of the interference conditions relatively different from each other by 180 degrees, respectively.
In the case of a coherent optical mixer circuit that is a light demodulation circuit in the related art, the chip connected to the delay circuit as shown in FIG. 2 is prepared. The transmission spectra as shown in FIGS. 5A and 5B are measured and analyzed to thereby evaluate the phase error and to select a favorable chip. Since the delay circuit is not used in an optical reception circuit, the chip is cut apart along the cut away standard line 9211 of FIG. 2 after the evaluation and selection, and is used for the optical reception circuit.